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28 Feb 2011

Volume 134, Issue 8, Articles (08xxxx)

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J. Chem. Phys. 134, 085101 (2011); http://dx.doi.org/10.1063/1.3552946 (22 pages)

Thomas E. Ouldridge, Ard A. Louis, and Jonathan P. K. Doye
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Communication: Generalizing Rosenfeld's excess-entropy scaling to predict long-time diffusivity in dense fluids of Brownian particles: From hard to ultrasoft interactions

Mark J. Pond, Jeffrey R. Errington, and Thomas M. Truskett

J. Chem. Phys. 134, 081101 (2011); http://dx.doi.org/10.1063/1.3559676 (4 pages) | Cited 7 times

Online Publication Date: 23 February 2011

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Computer simulations are used to test whether a recently introduced generalization of Rosenfeld's excess-entropy scaling method for estimating transport coefficients in systems obeying molecular dynamics can be extended to predict long-time diffusivities in fluids of particles undergoing Brownian dynamics in the absence of interparticle hydrodynamic forces. Model fluids with inverse-power-law, Gaussian-core, and Hertzian pair interactions are considered. Within the generalized Rosenfeld scaling method, long-time diffusivities of ultrasoft Gaussian-core and Hertzian particle fluids, which display anomalous trends with increasing density, are predicted (to within 20%) based on knowledge of interparticle interactions, excess entropy, and scaling behavior of simpler inverse-power-law fluids.
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61.20.Ja Computer simulation of liquid structure
05.40.Jc Brownian motion
66.10.C- Diffusion and thermal diffusion
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Communication: Shifted forces in molecular dynamics

Søren Toxvaerd and Jeppe C. Dyre

J. Chem. Phys. 134, 081102 (2011); http://dx.doi.org/10.1063/1.3558787 (4 pages) | Cited 1 time

Online Publication Date: 25 February 2011

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Simulations involving the Lennard-Jones potential usually employ a cutoff at r = 2.5σ. This communication investigates the possibility of reducing the cutoff. Two different cutoff implementations are compared, the standard shifted potential cutoff and the less commonly used shifted forces cutoff. The first has correct forces below the cutoff, whereas the shifted forces cutoff modifies Newton's equations at all distances. The latter is nevertheless superior; we find that for most purposes realistic simulations may be obtained using a shifted forces cutoff at r = 1.5σ, even though the pair force is here 30 times larger than at r = 2.5σ.
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34.20.Cf Interatomic potentials and forces
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Communication: A new approach to dual-basis second-order Møller–Plesset calculations

Jia Deng and Peter M. W. Gill

J. Chem. Phys. 134, 081103 (2011); http://dx.doi.org/10.1063/1.3556705 (4 pages)

Online Publication Date: 28 February 2011

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We describe a hierarchy of approximations (MP2[x]) that allow one to estimate second-order Møller–Plesset (MP2) energies in a large basis set from small-basis calculations. The most cost-effective approximation, MP2[K], is significantly cheaper than full MP2 but numerical tests on small atoms and molecules indicate that it is nonetheless accurate. We conclude that MP2[K] is an attractive level of theory for large systems.
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31.15.xp Perturbation theory
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back to top Theoretical Methods and Algorithms

Modeling molecular effects on plasmon transport: Silver nanoparticles with tartrazine

Christopher Arntsen, Kenneth Lopata, Michael R. Wall, Lizette Bartell, and Daniel Neuhauser

J. Chem. Phys. 134, 084101 (2011); http://dx.doi.org/10.1063/1.3541820 (7 pages) | Cited 1 time

Online Publication Date: 23 February 2011

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Modulation of plasmon transport between silver nanoparticles by a yellow fluorophore, tartrazine, is studied theoretically. The system is studied by combining a finite-difference time-domain Maxwell treatment of the electric field and the plasmons with a time-dependent parameterized method number 3 simulation of the tartrazine, resulting in an effective Maxwell/Schrödinger (i.e., classical/quantum) method. The modeled system has three linearly arranged small silver nanoparticles with a radius of 2 nm and a center-to-center separation of 4 nm; the molecule is centered between the second and third nanoparticles. We initiate an x-polarized current on the first nanoparticle and monitor the transmission through the system. The molecule rotates much of the x-polarized current into the y-direction and greatly reduces the overall transmission of x-polarized current.
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73.22.Lp Collective excitations
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.68.+m Optical properties of surfaces
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Design of effective kernels for spectroscopy and molecular transport: Time-dependent current–density-functional theory

Matteo Gatti

J. Chem. Phys. 134, 084102 (2011); http://dx.doi.org/10.1063/1.3558738 (5 pages)

Online Publication Date: 24 February 2011

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Time-dependent current–density-functional theory (TDCDFT) provides an, in principle, exact scheme to calculate efficiently response functions for a very broad range of applications. However, the lack of approximations valid for a range of parameters met in experimental conditions has so far delayed its extensive use in inhomogeneous systems. On the other side, in many-body perturbation theory accurate approximations are available, but at a price of a higher computational cost. In the present work, the possibility of combining the advantages of both approaches is exploited. In this way, an exact equation for the exchange-correlation kernel of TDCDFT is obtained, which opens the way for a systematic improvement of the approximations adopted in practical applications. Finally, an approximate kernel for an efficient calculation of spectra of solids and molecular conductances is suggested and its validity is discussed.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.70.Gm Exchange interactions
71.15.-m Methods of electronic structure calculations

Origin of anomeric effect: A density functional steric analysis

Ying Huang, Ai-Guo Zhong, Qinsong Yang, and Shubin Liu

J. Chem. Phys. 134, 084103 (2011); http://dx.doi.org/10.1063/1.3555760 (9 pages) | Cited 5 times

Online Publication Date: 24 February 2011

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The anomeric effect (the tendency of heteroatomic substituents adjacent to a heteroatom within the cyclohexane ring to prefer the axial orientation instead of the sterically less hindered equatorial position) is traditionally explained through either the dipole moment repulsion or the hyperconjugation effect. In this work, by employing our recent work in density functional steric analysis, we provide a novel two-component explanation, which is consistent with the common belief in chemistry that the effect has a stereoelectronic origin. With α-D-glucopyranose as the prototype, we systematically explore its conformational space and generate 32 isomers, leading to a total of 80 axial–equatorial conformation pairs. The energy difference analysis of these pairs shows that while statistically speaking the tendency is valid, the anomeric effect is not always true and can be violated. Three energy components, exchange–correlation, classical electrostatic, and density functional steric, are found to be directly proportional to the total energy difference between axial and equatorial isomers. We also found that the total dipole moment change, not the hyperconjugation effect, is a reasonable indicator of the total energy difference. However, all these correlations alone are not strong enough to provide a compellingly convincing explanation for the general validity of the effect. With the help of strong correlations between energy components, an explanation with two energy components, steric and electrostatic, was proposed in this work. We show that the axial–equatorial energy difference in general, with the anomeric effect as a special case, is dictated by two factors of the stereoelectronic origin, steric hindrance and classical electrostatic interactions, synchronously working together. Another explanation in terms of exchange–correlation and electrostatic interactions has also been obtained in this work.
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33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.eg Exchange-correlation functionals (in current density functional theory)
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Accelerating the convergence of path integral dynamics with a generalized Langevin equation

Michele Ceriotti, David E. Manolopoulos, and Michele Parrinello

J. Chem. Phys. 134, 084104 (2011); http://dx.doi.org/10.1063/1.3556661 (9 pages) | Cited 1 time

Online Publication Date: 24 February 2011

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The quantum nature of nuclei plays an important role in the accurate modelling of light atoms such as hydrogen, but it is often neglected in simulations due to the high computational overhead involved. It has recently been shown that zero-point energy effects can be included comparatively cheaply in simulations of harmonic and quasiharmonic systems by augmenting classical molecular dynamics with a generalized Langevin equation (GLE). Here we describe how a similar approach can be used to accelerate the convergence of path integral (PI) molecular dynamics to the exact quantum mechanical result in more strongly anharmonic systems exhibiting both zero point energy and tunnelling effects. The resulting PI-GLE method is illustrated with applications to a double-well tunnelling problem and to liquid water.
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02.70.Ns Molecular dynamics and particle methods
02.60.Lj Ordinary and partial differential equations; boundary value problems

Quantum Monte Carlo study of the first-row atoms and ions

P. Seth, P. López Ríos, and R. J. Needs

J. Chem. Phys. 134, 084105 (2011); http://dx.doi.org/10.1063/1.3554625 (8 pages) | Cited 2 times

Online Publication Date: 24 February 2011

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Quantum Monte Carlo calculations of the first-row atoms Li–Ne and their singly positively charged ions are reported. Multideterminant-Jastrow-backflow trial wave functions are used which recover more than 98% of the correlation energy at the variational Monte Carlo level and more than 99% of the correlation energy at the diffusion Monte Carlo level for both the atoms and ions. We obtain the first ionization potentials to chemical accuracy. We also report scalar relativistic corrections to the energies, mass-polarization terms, and one- and two-electron expectation values.
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31.15.xt Variational techniques
02.70.Uu Applications of Monte Carlo methods
32.50.+d Fluorescence, phosphorescence (including quenching)
31.30.jc Relativistic corrections to atomic structure and properties

Two-photon ionization of helium studied with the multiconfigurational time-dependent Hartree–Fock method

David Hochstuhl and Michael Bonitz

J. Chem. Phys. 134, 084106 (2011); http://dx.doi.org/10.1063/1.3553176 (10 pages)

Online Publication Date: 24 February 2011

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The multiconfigurational time-dependent Hartree–Fock method (MCTDHF) is applied for simulations of the two-photon ionization of helium. We present results for the single and double ionizations from the ground state for photon energies in the nonsequential regime and compare them to direct solutions of the Schrödinger equation using the time-dependent (full) configuration interaction (TDCI) method. We find that the single ionization is accurately reproduced by MCTDHF, whereas the double ionization results correctly capture the main trends of TDCI.
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31.15.xr Self-consistent-field methods
32.80.Fb Photoionization of atoms and ions

Density-functional approaches to noncovalent interactions: A comparison of dispersion corrections (DFT-D), exchange-hole dipole moment (XDM) theory, and specialized functionals

Lori A. Burns, Álvaro Vázquez- Mayagoitia, Bobby G. Sumpter, and C. David Sherrill

J. Chem. Phys. 134, 084107 (2011); http://dx.doi.org/10.1063/1.3545971 (25 pages) | Cited 31 times

Online Publication Date: 25 February 2011

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A systematic study of techniques for treating noncovalent interactions within the computationally efficient density functional theory (DFT) framework is presented through comparison to benchmark-quality evaluations of binding strength compiled for molecular complexes of diverse size and nature. In particular, the efficacy of functionals deliberately crafted to encompass long-range forces, a posteriori DFT+dispersion corrections (DFT-D2 and DFT-D3), and exchange-hole dipole moment (XDM) theory is assessed against a large collection (469 energy points) of reference interaction energies at the CCSD(T) level of theory extrapolated to the estimated complete basis set limit. The established S22 [revised in J. Chem. Phys. 132, 144104 (2010)] and JSCH test sets of minimum-energy structures, as well as collections of dispersion-bound (NBC10) and hydrogen-bonded (HBC6) dissociation curves and a pairwise decomposition of a protein–ligand reaction site (HSG), comprise the chemical systems for this work. From evaluations of accuracy, consistency, and efficiency for PBE-D, BP86-D, B97-D, PBE0-D, B3LYP-D, B970-D, M05-2X, M06-2X, ωB97X-D, B2PLYP-D, XYG3, and B3LYP-XDM methodologies, it is concluded that distinct, often contrasting, groups of these elicit the best performance within the accessible double-ζ or robust triple-ζ basis set regimes and among hydrogen-bonded or dispersion-dominated complexes. For overall results, M05-2X, B97-D3, and B970-D2 yield superior values in conjunction with aug-cc-pVDZ, for a mean absolute deviation of 0.41 – 0.49 kcal/mol, and B3LYP-D3, B97-D3, ωB97X-D, and B2PLYP-D3 dominate with aug-cc-pVTZ, affording, together with XYG3/6-311+G(3df,2p), a mean absolute deviation of 0.33 – 0.38 kcal/mol.
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31.15.E- Density-functional theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Fm Bond strengths, dissociation energies
87.14.E- Proteins
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Quantum Monte Carlo with Jastrow-valence-bond wave functions

Benoît Braïda, Julien Toulouse, Michel Caffarel, and C. J. Umrigar

J. Chem. Phys. 134, 084108 (2011); http://dx.doi.org/10.1063/1.3555821 (11 pages) | Cited 1 time

Online Publication Date: 25 February 2011

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We consider the use in quantum Monte Carlo calculations of two types of valence bond wave functions based on strictly localized active orbitals, namely valence bond self-consistent-field and breathing-orbital valence bond wave functions. Complemented by a Jastrow factor, these Jastrow-valence-bond wave functions are tested by computing the equilibrium well depths of the four diatomic molecules C2, N2, O2, and F2 in both variational Monte Carlo and diffusion Monte Carlo. We show that it is possible to design compact wave functions based on chemical grounds that are capable of describing both static and dynamic electron correlations. These wave functions can be systematically improved by inclusion of valence bond structures corresponding to additional bonding patterns.
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31.15.xw Valence bond calculations
31.15.V- Electron correlation calculations for atoms, ions and molecules

Look before you leap: A confidence-based method for selecting species criticality while avoiding negative populations in τ-leaping

Christian A. Yates and Kevin Burrage

J. Chem. Phys. 134, 084109 (2011); http://dx.doi.org/10.1063/1.3554385 (13 pages)

Online Publication Date: 25 February 2011

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The stochastic simulation algorithm was introduced by Gillespie and in a different form by Kurtz. There have been many attempts at accelerating the algorithm without deviating from the behavior of the simulated system. The crux of the explicit τ-leaping procedure is the use of Poisson random variables to approximate the number of occurrences of each type of reaction event during a carefully selected time period, τ. This method is acceptable providing the leap condition, that no propensity function changes “significantly” during any time-step, is met. Using this method there is a possibility that species numbers can, artificially, become negative. Several recent papers have demonstrated methods that avoid this situation. One such method classifies, as critical, those reactions in danger of sending species populations negative. At most, one of these critical reactions is allowed to occur in the next time-step. We argue that the criticality of a reactant species and its dependent reaction channels should be related to the probability of the species number becoming negative. This way only reactions that, if fired, produce a high probability of driving a reactant population negative are labeled critical. The number of firings of more reaction channels can be approximated using Poisson random variables thus speeding up the simulation while maintaining the accuracy. In implementing this revised method of criticality selection we make use of the probability distribution from which the random variable describing the change in species number is drawn. We give several numerical examples to demonstrate the effectiveness of our new method.
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82.20.Uv Stochastic theories of rate constants
82.20.Wt Computational modeling; simulation
82.30.-b Specific chemical reactions; reaction mechanisms

A simple, efficient polarizable coarse-grained water model for molecular dynamics simulations

Sereina Riniker and Wilfred F. van Gunsteren

J. Chem. Phys. 134, 084110 (2011); http://dx.doi.org/10.1063/1.3553378 (12 pages) | Cited 5 times

Online Publication Date: 28 February 2011

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The development of coarse-grained (CG) models that correctly represent the important features of compounds is essential to overcome the limitations in time scale and system size currently encountered in atomistic molecular dynamics simulations. Most approaches reported in the literature model one or several molecules into a single uncharged CG bead. For water, this implicit treatment of the electrostatic interactions, however, fails to mimic important properties, e.g., the dielectric screening. Therefore, a coarse-grained model for water is proposed which treats the electrostatic interactions between clusters of water molecules explicitly. Five water molecules are embedded in a spherical CG bead consisting of two oppositely charged particles which represent a dipole. The bond connecting the two particles in a bead is unconstrained, which makes the model polarizable. Experimental and all-atom simulated data of liquid water at room temperature are used for parametrization of the model. The experimental density and the relative static dielectric permittivity were chosen as primary target properties. The model properties are compared with those obtained from experiment, from clusters of simple-point-charge water molecules of appropriate size in the liquid phase, and for other CG water models if available. The comparison shows that not all atomistic properties can be reproduced by a CG model, so properties of key importance have to be selected when coarse graining is applied. Yet, the CG model reproduces the key characteristics of liquid water while being computationally 1–2 orders of magnitude more efficient than standard fine-grained atomistic water models.
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61.20.Ja Computer simulation of liquid structure
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Fm Bond strengths, dissociation energies
36.40.Wa Charged clusters

Molecular cluster building algorithm: Electrostatic guidelines and molecular tailoring approach

Sachin D. Yeole and Shridhar R. Gadre

J. Chem. Phys. 134, 084111 (2011); http://dx.doi.org/10.1063/1.3556819 (9 pages) | Cited 3 times

Online Publication Date: 28 February 2011

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Nano-sized clusters of various materials are recent experimental targets, since they exhibit size-dependent physico-chemical properties. A vast amount of literature is available on the study of molecular clusters but general methods for systematic evolution of their growth are rather scarce. The present work reports a molecular cluster building algorithm based on the electrostatic guidelines, followed by ab initio investigations, enabled by the application of molecular tailoring approach. Applications of the algorithm for generating geometries and interaction energies of large molecular clusters of zinc sulfide, benzene, and water are presented.
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31.15.ap Polarizabilities and other atomic and molecular properties
36.40.-c Atomic and molecular clusters

Reformulated space-charge-limited current model and its application to disordered organic systems

Cristiano F. Woellner and José A. Freire

J. Chem. Phys. 134, 084112 (2011); http://dx.doi.org/10.1063/1.3548884 (10 pages) | Cited 1 time

Online Publication Date: 28 February 2011

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We have reformulated a traditional model used to describe the current–voltage dependence of low mobility materials sandwiched between planar electrodes by using the quasi-electrochemical potential as the fundamental variable instead of the local electric field or the local charge carrier density. This allows the material density-of-states to enter explicitly in the equations and dispenses with the need to assume a particular type of contact. The diffusion current is included and as a consequence the current–voltage dependence obtained covers, with increasing bias, the diffusion limited current, the space-charge limited current, and the injection limited current regimes. The generalized Einstein relation and the field and density dependent mobility are naturally incorporated into the formalism; these two points being of particular relevance for disordered organic semiconductors. The reformulated model can be applied to any material where the carrier density and the mobility may be written as a function of the quasi-electrochemical potential. We applied it to the textbook example of a nondegenerate, constant mobility material and showed how a single dimensionless parameter determines the form of the I(V) curve. We obtained integral expressions for the carrier density and for the mobility as a function of the quasi-electrochemical potential for a Gaussianly disordered organic material and found the general form of the I(V) curve for such materials over the full range of bias, showing how the energetic disorder alone can give rise, in the space-charge limited current regime, to an I∝Vn dependence with an exponent n larger than 2.
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72.20.Ht High-field and nonlinear effects
82.45.-h Electrochemistry and electrophoresis
72.80.Ng Disordered solids
72.80.Le Polymers; organic compounds (including organic semiconductors)
66.30.-h Diffusion in solids
72.20.Fr Low-field transport and mobility; piezoresistance
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Intensity analysis of overlapped discrete and continuous absorption by spectral simulation: The electronic transition moment for the B–X system in I2

Joel Tellinghuisen

J. Chem. Phys. 134, 084301 (2011); http://dx.doi.org/10.1063/1.3555623 (8 pages) | Cited 1 time

Online Publication Date: 22 February 2011

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The spectrum of I2 is examined anew in the wavelength region 520–640 nm, where discrete absorption in the B–X transition is prominent. The spectrum is recorded with high quantitative precision at moderate resolution (0.1 nm) and is analyzed by least-squares spectral simulation, yielding the B–X electronic transition strength |μe|2 with unprecedented precision (<2% relative standard error) over most of the studied region. The analysis also yields directly new estimates of the continuous absorption in this region, which support previous assessments of the A ← X transition but lower the C(1Πu) ← X transition strength by 25%. The new analysis method is applicable to any situation where the discrete spectrum can be simulated reliably.
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33.20.Kf Visible spectra
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Infrared spectroscopy of small protonated water clusters at room temperature: An effective modes analysis

Federica Agostini, Rodolphe Vuilleumier, and Giovanni Ciccotti

J. Chem. Phys. 134, 084302 (2011); http://dx.doi.org/10.1063/1.3533229 (18 pages) | Cited 1 time

Online Publication Date: 22 February 2011

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We perform infrared vibrational analysis on small protonated water clusters H+(H2O)n, with n = 2, …, 6, at room temperature. The absorption spectra are calculated based on classical trajectories obtained by the multistate empirical valence bond method. The analysis is carried out based on the effective modes analysis, which has been recently developed [Martinez et al., J. Chem. Phys. 125, 144106 (2006)] as generalization of the normal modes analysis. This technique enables us to decompose the full spectrum in maximally localized bands which are obtained by accounting for temperature and anharmonic effects. These effects are especially considered in the determination of the modes coupling. The spectra of the small clusters are interpreted by identifying the behavior of the excess charge, by understanding the role of hydrogen bonds, and by considering the effect of (micro-)solvation. Our results are presented by showing comparisons with other numerical methods and experimental measurements which are available in the literature.
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36.40.Cg Electronic and magnetic properties of clusters
33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis
33.15.Fm Bond strengths, dissociation energies
31.15.xw Valence bond calculations

Infrared spectroscopy and effective modes analysis of the protonated water dimer H+(H2O)2 at room temperature under H/D substitution

Federica Agostini, Rodolphe Vuilleumier, and Giovanni Ciccotti

J. Chem. Phys. 134, 084303 (2011); http://dx.doi.org/10.1063/1.3521273 (10 pages) | Cited 1 time

Online Publication Date: 22 February 2011

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We study the vibrational properties of the protonated water dimer and its deuterated forms at room temperature. Molecular dynamics simulations within the empirical valence bond (EVB) model are used to generate the vibrational spectra that are interpreted using the effective modes analysis (EMA). Quantum effects are taken into account through an effective parametrization of the EVB model. EMA allows for the assignment of the bands in the 1000 − 2000 cm−1 region of the protonated water dimer from the molecular dynamics trajectory. It is then found that although this system is very anharmonic the two main bands in this spectral region arise from a linear coupling between the asymmetric OH+O stretch and asymmetric bend of the two water molecules. This mixing explains the simulated band shifts upon isotopic substitution of the central proton or of the hydrogens of the two water molecules.
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33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.xw Valence bond calculations
31.15.xv Molecular dynamics and other numerical methods

Phase space geometry of dynamics passing through saddle coupled with spatial rotation

Shinnosuke Kawai and Tamiki Komatsuzaki

J. Chem. Phys. 134, 084304 (2011); http://dx.doi.org/10.1063/1.3554906 (11 pages) | Cited 1 time

Online Publication Date: 22 February 2011

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Nonlinear reaction dynamics through a rank-one saddle is investigated for many-particle system with spatial rotation. Based on the recently developed theories of the phase space geometry in the saddle region, we present a theoretical framework to incorporate the spatial rotation which is dynamically coupled with the internal vibrational motions through centrifugal and Coriolis interactions. As an illustrative simple example, we apply it to isomerization reaction of HCN with some nonzero total angular momenta. It is found that no-return transition state (TS) and a set of impenetrable reaction boundaries to separate the “past” and “future” of trajectories can be identified analytically under rovibrational couplings. The three components of the angular momentum are found to have distinct effects on the migration of the “anchor” of the TS and the reaction boundaries through rovibrational couplings and anharmonicities in vibrational degrees of freedom. This method provides new insights in understanding the origin of a wide class of reactions with nonzero angular momentum.
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82.30.Qt Isomerization and rearrangement
33.20.Vq Vibration-rotation analysis
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Vibrational spectrum of Ar3+ and relative importance of linear and perpendicular isomers in its photodissociation

František Karlický, Bruno Lepetit, René Kalus, and Florent Xavier Gadéa

J. Chem. Phys. 134, 084305 (2011); http://dx.doi.org/10.1063/1.3555275 (7 pages)

Online Publication Date: 22 February 2011

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The photodissociation dynamics of the argon ionized trimer Ar 3+ is revisited in the light of recent experimental results of Lepère et al. [J. Chem. Phys. 134, 194301 (2009)], which show that the fragment with little kinetic energy is always a neutral one, thus the available energy is shared by a neutral and ionic fragments as in Ar 2+. We show that these results can be interpreted as the photodissociation of the linear isomer of the system. We perform a 3D quantum computation of the vibrational spectrum of the system and study the relative populations of the linear (trimer-core) and perpendicular (dimer-core) isomers. We then show that the charge initially located on the central atom in the ground electronic state of the linear isomer migrates toward the extreme ones in the photoexcitation process such that photodissociation of the linear isomer produces a neutral central atom at rest in agreement with measured product state distributions.
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36.40.Wa Charged clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Some consequences of high temperature water vapor spectroscopy: Water dimer at equilibrium

M. Yu. Tretyakov and D. S. Makarov

J. Chem. Phys. 134, 084306 (2011); http://dx.doi.org/10.1063/1.3556606 (9 pages) | Cited 3 times

Online Publication Date: 23 February 2011

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It is shown that the evolution of water vapor spectra in the 2500–5000 cm−1 range recorded at 650 K and pressures up to 130 atms after subtraction of monomer contribution may be interpreted qualitatively well on the basis of experimental data on water dimer and trimer obtained from cold molecular beams and in He droplets. The proposed spectroscopic model considers water vapor as a mixture of nonideal monomers, dimers, and trimers at chemical equilibrium. The effect of line mixing is taken into account in the monomer spectrum modeling. Decomposition of the high temperature spectra allowed determining a dimer equilibrium constant that was compared with the previously known values. The contribution of water trimer is assessed. The performed analysis indicates that the number of bound dimers in water vapor is quite large, even at such a high temperature.
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33.20.Vq Vibration-rotation analysis
82.60.Hc Chemical equilibria and equilibrium constants

A first-principles study of the influence of helium atoms on the optical response of small silver clusters

M. Pereiro, D. Baldomir, and J. E. Arias

J. Chem. Phys. 134, 084307 (2011); http://dx.doi.org/10.1063/1.3556821 (7 pages)

Online Publication Date: 24 February 2011

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Optical excitation spectra of Agn and Agn@He60 (n = 2, 8) clusters are investigated in the framework of the time-dependent density functional theory (TDDFT) within the linear response regime. We have performed the ab initio calculations for two different exact exchange functionals (GGA-exact and LDA-exact). The computed spectra of Agn@He60 clusters with the GGA-exact functional accounting for exchange-correlation effects are found to be generally in a relatively good agreement with the experiment. A strategy is proposed to obtain the ground-state structures of the Agn@He60 clusters and in the initial process of the geometry optimization, the He environment is simulated with buckyballs. A redshift of the silver clusters spectra is observed in the He environment with respect to the ones of bare silver clusters. This observation is discussed and explained in terms of a contraction of the Ag–He bonding length and a consequent confinement of the s valence electrons in silver clusters. Likewise, the Mie–Gans predictions combined with our TDDFT calculations also show that the dielectric effect produced by the He matrix is considerably less important in explaining the redshifting observed in the optical spectra of Agn@He60 clusters.
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36.40.Vz Optical properties of clusters
33.20.Kf Visible spectra
33.70.Jg Line and band widths, shapes, and shifts
31.15.ee Time-dependent density functional theory
31.15.A- Ab initio calculations

Ab initio spectroscopic characterization of the HNNO and ONHN radicals

Kirk A. Peterson and Joseph S. Francisco

J. Chem. Phys. 134, 084308 (2011); http://dx.doi.org/10.1063/1.3556990 (10 pages) | Cited 1 time

Online Publication Date: 24 February 2011

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A composite coupled cluster methodology is used with systematic sequences of correlation consistent basis sets to accurately determine the structure, vibrational frequencies, and isotopic shifts for trans-HNNO (2A′), cis-HNNO (2A′), and ONHN (2A′). Anharmonic corrections to the vibrational frequencies and rotational constants are obtained using density functional theory. With basis sets larger than double-zeta, large differences between restricted open-shell Hartree–Fock (ROHF)-based and unrestricted Hartree–Fock (UHF)-based coupled cluster harmonic frequencies are calculated, with the UHF-based ones judged to be more reliable based on an analysis of the orbital hessian eigenvalues. The final calculated anharmonic vibrational band origins are generally in good agreement with the experimental values measured in rare gas matrices. The calculation of the vibrational band origins of the isovalent NO2 molecule at similar levels of theory exhibits an agreement with experiment to within a few wavenumbers. In the latter case, however, a ROHF treatment was required since the UHF approach failed to provide realistic frequencies for the antisymmetric stretching mode. The heat of formation at 0 K of trans-HNNO is calculated to be 50.5 ± 0.5 kcal/mol using a very similar composite coupled cluster methodology as in the structure and harmonic frequency determinations.
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31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.70.Jg Line and band widths, shapes, and shifts
31.15.E- Density-functional theory

The electronic states of 1,2,3-triazole studied by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy, and a comparison with ab initio configuration interaction methods

Michael H. Palmer, Søren Vrønning Hoffmann, Nykola C. Jones, Ashley R. Head, and Dennis L. Lichtenberger

J. Chem. Phys. 134, 084309 (2011); http://dx.doi.org/10.1063/1.3549812 (13 pages) | Cited 1 time

Online Publication Date: 24 February 2011

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The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ [5s3p3d1f] basis set and a set of Rydberg [4s3p3d3f] functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (∼5.5–6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved resolution yielded adiabatic and vertical ionization energies and reorganization energies for several of the lowest cationic states. As well as excitations to the s, p, d-Rydberg states are the excitations consistent with an f-series.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.60.+q Photoelectron spectra
31.15.bw Coupled-cluster theory
31.15.vq Electron correlation calculations for polyatomic molecules
31.15.xr Self-consistent-field methods
33.20.Ni Vacuum ultraviolet spectra

Exploring single and double proton transfer processes in the gas phase: A high resolution electronic spectroscopy study of 5-fluorosalicylic acid

Justin W. Young, Adam J. Fleisher, and David W. Pratt

J. Chem. Phys. 134, 084310 (2011); http://dx.doi.org/10.1063/1.3554431 (8 pages) | Cited 1 time

Online Publication Date: 25 February 2011

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Two species that possess different absorption and emission properties have been observed in the low resolution fluorescence excitation spectrum of 5-fluorosalicylic acid (FSA) in the gas phase. The two species were identified as monomer and dimer species using high resolution techniques. Studies of these spectra in the presence of an applied electric field, together with ab initio quantum chemistry calculations, show that the monomer is a “closed” form of FSA exhibiting an intramolecular C = O⋅⋅⋅H–O–C hydrogen bond in the ground state. Absorption of light at ∼344 nm transforms this species into the tautomeric form C–O–H⋅⋅⋅O = C via a barrierless proton transfer process. The large charge rearrangement that accompanies this process results in a significantly red-shifted emission spectrum. The (FSA)2 dimer exhibits two intermolecular C = O⋯H–O–C hydrogen bonds but in this case the double proton transfer leads to a conical intersection with the ground state and rapid nonradiative decay. The onset of this process and the time scale on which it occurs are revealed by a homogeneous broadening of the dimer's high resolution spectrum.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
33.20.Lg Ultraviolet spectra
31.15.A- Ab initio calculations
33.50.Dq Fluorescence and phosphorescence spectra
33.70.Jg Line and band widths, shapes, and shifts
33.15.Fm Bond strengths, dissociation energies
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